The classic radiographic or "X-ray" image is obtained by situating an object to be imaged between an X-ray emitter (i.e., an X-ray tube) and an X-ray detector. Emitted X-rays pass through the object to strike the detector, with the response of the detector varying over its area as a function of the intensity of the incident X-rays. Since the intensity of the X-rays incident on the detector is largely a function of the density of the object along the path of the X-rays, the detector receives a shadow image of the object which may then be viewed and analyzed by X-ray technicians, e.g., radiologists. In the case of analog radiographic systems, the detector is formed of X-ray film, whereas digital radiographic systems have solid-state detector components (e.g., scintillator/photodiode arrays) whereby the image is provided in electronic form.
One difficulty which is commonly encountered with the analysis of radiographic images is the proper identification of objects contained within the image. As an example, the identification of organs and other body structures is particularly important in radiographic thoracic imaging (the taking of chest X-rays). In the most common type of chest X-ray, a patient will place his/her chest against a detector and the emitter will be activated to send X-rays through the patient from the posterior-to-anterior direction and into the detector. When the image is captured, a radiologist must then systematically evaluate the image to identify the chest wall, diaphragm, lungs, pleura, mediastinum, etc. To properly identify and analyze matters of medical importance, it is desirable to be able to identify extremely small objects on the image, e.g., details as small as 0.7-2.0 mm near the center of the lungs and 0.3-2.0 mm near their periphery. However, it is difficult for a radiologist to identify objects this small on a two-dimensional image, particularly since some objects may be overlapping and their boundaries may be difficult to accurately discern.